Cultured rat basophilic leukemia (RBL-2H3) cells previously labeled with myo-(2-3H)inositol for 24 hr showed an appreciable release of (3H) inositiol phosphates and a reciprocal decline in (3H) inositol phospholipids upon exposure to cobra cardiotoxin (Naja naja kaauthia). The most significant effect was a selective increase in the amount of 3H-label in the phosphotidyl inositol monophosphate (PIP) pool. The extent of increase was dependent on concentration and time of exposure to cardiotoxin. The kinetics of redistribution of label within the various inositol lipid and inositol phosphate pools suggested that a major action was the stimulation of phosphatidylinositol kinase activity which converts phosphatidylinositol (PI) to PIP. This action was confirmed by assay of the enzyme activity in membrane preparations. Cardiotoxin also caused a time- and concentration-dependent secretion of histamine and release of lactate dehydrogenase (LDH) from the 2H3 cells. The release of histamine but not LDH was totally dependent on external calcium. The results suggest that cardiotoxin-stimulated release of histamine from 2H3 cells is through a CA(2+)-dependent noncytotoxic process.